Preparation And Electrocatalytic Properties Of Transition Metal Cobalt-based Phosphide Composites

Hydrogen energy has been widely studied because of its advantages of large combustion calorific value,renewable,clean and pollution-free combustion products,and various preparation methods.At present,the blueprint of world hydrogen energy economy is gradually taking shape.Compared with the traditional hydrogen production method of methane steam reforming with high energy consumption,electrocatalytic water splitting hydrogen production has become one of the most potential hydrogen production technologies because of its high theoretical energy conversion efficiency,mild reaction conditions,simple device and high purity hydrogen production.High-performance electrocatalysts are very important to reduce the energy consumption of hydrogen production by electrolysis of water.At present,noble metal-based catalysts show the best performance,but the rarity of noble metals limits the further development of hydrogen production by electrolysis of water.Therefore,rational design and development of electrocatalysts with low cost,abundant reserves,high efficiency and high stability are of great significance to the development of hydrogen economy.In view of this,transition metal iron,cobalt and vanadium are selected as the research objects in this paper,aiming at developing low-cost and high-activity transition metal cobalt-based phosphide electrocatalysts.Taking advantage of the advantages of large specific surface area and adjustable porosity of metal-organic framework（MOFs）materials,a variety of transition metal cobalt-based phosphide composites were prepared successfully by heterointerface construction,morphology control,porous structure construction and in-situ growth of catalysts on conductive substrates,and the catalytic properties for hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）and overall water splitting（OWS）were studied systematically.The study includes the following three parts:（1）Preparation and studies of heterostructure CoP₃/Fe₂P nanosheets for electrocatalytic water splitting:the Co P₃/Fe₂P nanosheet catalysts were successfully grown in situ on 3D NF conductive substrate through a hydrothermal-phosphating strategy,and then the electrocatalytic performance of catalysts was investigated systematically.Thanks to ultrathin nanosheet structure of Co P₃/Fe₂P,abundant heterogeneous interfaces,a large number of nanopores,and the unique 3D network structure of NF substrate,the Co P₃/Fe₂P@NF-2 catalyst simultaneously exhibited excellent HER and OER bifunctional electrocatalytic activity.At a current density of 10m A cm^-2,only 81 m V and 236 mV overpotentials were required for HER and OER in1.0 M KOH electrolyte,respectively.When Co P_3/Fe₂P@NF-2 catalyst was used as cathode and anode of electrolyzer to catalyze electrolysis of water,it merely required an operating voltage of 1.53 V to drive a current density of 20 m A cm^-2with prominent long-term stabilization,comparable to most reported catalysts.Density functional theory（DFT）calculation also proved that Co P₃-Fe₂P heterointerfaces can further optimize hydrogen adsorption energy(ΔG_H*)to effectively reduce free energy barrier.（2）Preparation of ZIF-coated Co₂P/V₃P composites and its application in overall water splitting:the ZIF-Co₂P/V₃P@NF electrocatalysts with hollow spherical structure were prepared by hydrothermal,static method and subsequent low-temperature gas phase phosphating.The optimized ZIF-Co₂P/V₃P@NF-2 catalyst showed the best catalytic performance and stability due to its hollow structure covered by ZIF skeleton,abundant heterogeneous interfaces and synergistic effect between Co₂P and V₃P phases.Catalyzing HER and OER in 1.0 M KOH,only 57 m V and 261 m V overpotentials were required when the current density reached 10 m A cm^-2,respectively.Moreover,an alkaline electrolyzer assembled with ZIF-Co₂P/V₃P@NF-2 catalyst as cathode and anode only needed 1.54 V decomposition voltage,which can drive the current density of10 m A cm^-2and showed excellent stability.（3）Preparation of MOF-derived Co/V bimetal phosphides and its application in electrocatalytic water splitting:MOF-derived Co/V bimetal phosphide bifunctional electrocatalysts（Co_xVP-MOF@NF）were prepared successfully on NF substrate by solvothermal method,alkali etching method and low temperature gas phase phosphating method.Thanks to the abundant active sites exposed by a large number of defects and the highly open network channels of the 3D NF conductive substrate,the Co₂VP-MOF@NF catalyst only needed 80 m V and 265 m V overpotentials for HER and OER at 10 m A cm^-2.More importantly,only a decomposition voltage of 1.51 V was needed in an alkaline electrolyzer,and accompanied by good electrochemical stability.